In recent years, infrared gas detectors have been developed for detection of gas density by sensing absorption of infrared light of specific wavelengths. For example, Japanese Patent Document JP-A-2003-50203 discloses an infrared gas detector having an infrared sensor for detecting absorption of infrared light.
The infrared gas detector (non-dispersive infrared gas detector) is equipped with multiple optical filters for filtering infrared light of specific wavelengths (passing infrared light of specific wavelengths). The filters are intended for filtering specific wavelength ranges of infrared light used to detect certain components of a sample gas. The infrared light is projected toward the gas in a sampling case, to be received by the infrared sensor after being filtered through the sample gas and the optical filters.
For example, water vapor density in a gas is affected by a carbon monoxide (CO) component. Therefore, when measuring a gas for water vapor density having CO, the CO interference in the measurement has to be corrected/removed by an optical filter in the infrared light path.
On the other hand, measuring a gas for CO density when water vapor is also present in the gas is performed in the following manner. The infrared light is filtered through an optical filter to introduce a wavelength range having a CO absorption rate and a relatively small water vapor absorption rate for the sample gas. Then, water vapor interference in the measurement is removed to have an accurate measurement of CO density. In this manner, density of a sample gas having infrared light absorption wavelength range similar to water vapor can be measured even in the presence of water vapor.
However, the optical filters have to be switched for a measurement of H2O and CO. In the disclosure described above, the H2O filter and the CO filter used for H2O measurement and CO measurement are disposed on a rotating chopper to cut the infrared light path alternately in a predetermined interval. In this case, the chopper is equipped with a position sensor to detect a filter position that is on the infrared light path at the moment, thereby making it difficult to enclose the infrared gas detector in a small package.